Antioxidants in the blood plasma of rats were measured as part of a comprehensive, multilaboratory validation study searching for non-invasive biomarkers of oxidative stress. For this initial study an animal model of CCl4 poisoning was studied. The time-(2, 7 and 16 h) and dose-(120 mg/kg and 1200 mg/kg, i. p.) dependent effects of CCl4 on plasma levels of alpha-tocopherol, ascorbic acid, glutathione (GSH and GSSG), uric acid and total antioxidant capacity were investigated to determine whether the oxidative effects of CCl4 would result in losses of antioxidants from plasma. The alpha-tocopherol concentration was decreased in CCl4-treated rats. However, due to concomitant decreases in cholesterol and triglycerides, it was impossible to dissociate oxidation of alpha-tocopherol from generalized lipid changes. Ascorbic acid levels were higher with treatment at the earliest time point; the ratio of GSH to GSSG generally declined, and uric acid remained unchanged. Total antioxidant capacity showed no significant change except for 16 h after the high dose, when it was increased. These results suggest that plasma changes due to liver malfunction and rupture of liver cells impede detection of any potential changes in the antioxidant status of the plasma.